A Newcastle virus strain, selected after several isolating experiments which produced a Epizootia in Argentina, was tested at 37, 46 and 56ºC, at different exposition times. Letal embryo 50% and hemagglutination tests were conducted in order to know the effect of both variables (temperature and time), and compared with controls without treatment. To determine the intracerebral pathogenicity index, chickens of 2 days of age were inoculated with the isolated Newcastle strain which was previously treated with a suspension of chicken nervous cells.